Cellular expression of alpha7 nicotinic acetylcholine receptor protein in the temporal cortex in Alzheimer's and Parkinson's disease--a stereological approach

Compositional analysis of Swertia chirayita medicinal plant using laser-induced breakdown spectroscopy and ICP-MS

One of the most significant medicinal plants used to treat numerous illnesses is Swertia chirayita. The present study demonstrated the compositional analysis of the Swertia chirayita (S. chirayita) plant using an emerging and non-destructive laser-induced breakdown spectroscopy (LIBS) technique. Mg, Ca, K, Fe, Sr, Cr, and Na were verified as necessary elements by the optical emission investigations, while Al, Ti, Si, Ba, Mn, and Li were non-essential. Using the Boltzmann plot technique with stark broadening parameters, plasma temperature and electron number density were calculated in the range of (10,000-12,000) K ±1000 K and (1.5-1.8) × 1017 cm-3, respectively. Finally, compositional analysis was carried out using calibration-free (CF-LIBS) analysis and results were compared with ICP-MS. It was observed that the concentration of Ca and Fe is higher than other detected elements. All the toxic elements are found to be within the safe limit. So, this medicinal plant can be used to cure a variety of diseases that arise due to the deficiency of these elements.

The role of astrocytic α7 nicotinic acetylcholine receptors in Alzheimer disease

The ongoing search for therapeutic interventions in Alzheimer disease (AD) has highlighted the complexity of this condition and the need for additional biomarkers, beyond amyloid-β (Aβ) and tau, to improve clinical assessment. Astrocytes are brain cells that control metabolic and redox homeostasis, among other functions, and are emerging as an important focus of AD research owing to their swift response to brain pathology in the initial stages of the disease. Reactive astrogliosis - the morphological, molecular and functional transformation of astrocytes during disease - has been implicated in AD progression, and the definition of new astrocytic biomarkers could help to deepen our understanding of reactive astrogliosis along the AD continuum. As we highlight in this Review, one promising biomarker candidate is the astrocytic α7 nicotinic acetylcholine receptor (α7nAChR), upregulation of which correlates with Aβ pathology in the brain of individuals with AD. We revisit the past two decades of research into astrocytic α7nAChRs to shed light on their roles in the context of AD pathology and biomarkers. We discuss the involvement of astrocytic α7nAChRs in the instigation and potentiation of early Aβ pathology and explore their potential as a target for future reactive astrocyte-based therapeutics and imaging biomarkers in AD.

Nicotinic acetylcholine receptors in neurological and psychiatric diseases

Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels that are widely distributed both pre- and post-synaptically in the mammalian brain. By modulating cation flux across cell membranes, neuronal nAChRs regulate neuronal excitability and the release of a variety of neurotransmitters to influence multiple physiologic and behavioral processes including synaptic plasticity, motor function, attention, learning and memory. Abnormalities of neuronal nAChRs have been implicated in the pathophysiology of neurologic disorders including Alzheimer's disease, Parkinson's disease, epilepsy, and Tourette´s syndrome, as well as psychiatric disorders including schizophrenia, depression, and anxiety. The potential role of nAChRs in a particular illness may be indicated by alterations in the expression of nAChRs in relevant brain regions, genetic variability in the genes encoding for nAChR subunit proteins, and/or clinical or preclinical observations where specific ligands showed a therapeutic effect. Over the past 25 years, extensive preclinical and some early clinical evidence suggested that ligands at nAChRs might have therapeutic potential for neurologic and psychiatric disorders. However, to date the only approved indications for nAChR ligands are smoking cessation and the treatment of dry eye disease. It has been argued that progress in nAChR drug discovery has been limited by translational gaps between the preclinical models and the human disease as well as unresolved questions regarding the pharmacological goal (i.e., agonism, antagonism or receptor desensitization) depending on the disease.

Nicotinic acetylcholine receptor subtype expression, function, and pharmacology: Therapeutic potential of α-conotoxins

The pentameric nicotinic acetylcholine receptors (nAChRs) are typically classed as muscle- or neuronal-type, however, the latter has also been reported in non-neuronal cells. Given their broad distribution, nAChRs mediate numerous physiological and pathological processes including synaptic transmission, presynaptic modulation of transmitter release, neuropathic pain, inflammation, and cancer. There are 17 different nAChR subunits and combinations of these subunits produce subtypes with diverse pharmacological properties. The expression and role of some nAChR subtypes have been extensively deciphered with the aid of knock-out models. Many nAChR subtypes expressed in heterologous systems are selectively targeted by the disulfide-rich α-conotoxins. α-Conotoxins are small peptides isolated from the venom of cone snails, and a number of them have potential pharmaceutical value.

The α7 nAChR allosteric modulator PNU-120596 amends neuroinflammatory and motor consequences of parkinsonism in rats: Role of JAK2/NF-κB/GSk3β/ TNF-α pathway

Parkinson's disease (PD) is the second most common neurodegenerative disorder and a leading cause of disability. The current gold standard for PD treatment, L-Dopa, has limited clinical efficacy and multiple side effects. Evidence suggests that activation of α7 nicotinic acetylcholine receptors (α7nAChRs) abrogates neuronal and inflammatory insults. Here we tested whether PNU-120596 (PNU), a type II positive allosteric modulator of α7 nAChR, has a critical role in regulating motor dysfunction and neuroinflammation correlated with the associated PD dysfunction. Neuroprotective mechanisms were investigated through neurobehavioral, molecular, histopathological, and immunohistochemical studies. PNU reversed motor incoordination and hypokinesia induced via the intrastriatal injection of 6-hydroxydopamine and manifested by lower falling latency in the rotarod test, short ambulation time and low rearing incidence in open field test. Tyrosine hydroxylase immunostaining showed a significant restoration of dopaminergic neurons following PNU treatment, in addition to histopathological restoration in nigrostriatal tissues. PNU halted striatal neuroinflammation manifested as a suppressed expression of JAK2/NF-κB/GSk3β accompanied by a parallel decline in the protein expression of TNF-α in nigrostriatal tissue denoting the modulator anti-inflammatory capacity. Moreover, the protective effects of PNU were partially reversed by the α7 nAChR antagonist, methyllycaconitine, indicating the role of α7 nAChR modulation in the mechanism of action of PNU. This is the first study to reveal the positive effects of PNU-120596 on motor derangements of PD via JAK2/NF-κB/GSk3β/ TNF-α neuroinflammatory pathways, which could offer a potential therapeutic strategy for PD.

Exposure to varenicline protects against locomotor alteration in a MPTP mouse model of Parkinson's disease

The beneficial effects of drugs that act via nicotinic acetylcholine receptors (nAChRs) on Parkinson's disease (PD) symptomatology may explain the negative correlation between cigarette smoking and risk of this neurological condition. Varenicline, an α4β2 nAChR partial agonist approved for smoking cessation treatments, could be valuable for PD treatment. Here, we investigated varenicline effects in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) PD mouse model. From postnatal day (PN) 90 to PN119, male C57BL/6 mice were exposed daily to varenicline (2 mg/kg) by gavage. After that, MPTP was injected (30 mg/kg, ip) once a day for five days. At PN125, locomotor and anxiety-like effects were assessed with the open field test. At PN126, immobile behavior was assessed with the forced swimming test. At PN127, the frontal cerebral cortex was collected to evaluate dopamine and DOPAC levels. To verify whether varenicline was protective during the MPTP insult, a separate group of MPTP animals received varenicline from PN90 to PN124. MPTP reduced cortical dopamine content and increased dopamine turnover. Those effects were not reversed by varenicline treatment. Interestingly, varenicline reversed the MPTP-induced hyperactivity in the open field. Both maintenance of varenicline treatment during MPTP exposure or its interruption before MPTP exposure elicited similar results. No alterations were observed in anxiety-like behavior or in immobility time. Altogether, these findings suggested that varenicline treatment reduced the MPTP-induced hyperactivity, but did not protect against dopaminergic damage. Based on this partial protective effect, varenicline could exert neuroprotective effects on circuits that control motor activity in PD.

nAChRs gene expression and neuroinflammation in APPswe/PS1dE9 transgenic mouse

An evaluation of the APPswe/PS1dE9 transgenic AD mouse, presenting with the toxic Aβ1-42 deposition found in human AD, allowed us to characterize time-dependent changes in inflammatory and cholinergic markers present in AD. Astrogliosis was observed in cortex and hippocampus, with cellular loss occurring in the same areas in which Aβ plaques were present. In this setting, we found early significantly elevated levels of IL-1β and TNFα gene expression; with the hippocampus showing the highest IL-1β expression. To investigate the cholinergic anti-inflammatory pathway, the expression of nicotinic receptors (nAChRs) and cholinesterase enzymes also was evaluated. The anti-inflammatory nAChRα7, α4, and β2 were particularly increased at 6 months of age in the hippocampus, potentially as a strategy to counteract Aβ deposition and the ensuing inflammatory state. A time-dependent subunit switch to the α3β4 type occurred. Whether α3, β4 subunits have a pro-inflammatory or an inhibitory effect on ACh stimulation remains speculative. Aβ1-42 deposition, neuronal loss and increased astrocytes were detected, and a time-dependent change in components of the cholinergic anti-inflammatory pathway were observed. A greater understanding of time-dependent Aβ/nAChRs interactions may aid in defining new therapeutic strategies and novel molecular targets.

COVID-19 and nicotine as a mediator of ACE-2

We recently reported that current smokers and those with COPD had higher airway epithelial cell expression of the angiotensin-converting enzyme II (ACE-2) viral entry receptor [1]. We thus read with great interest the work of P. Russo and co-workers, which proposes a mechanism for this finding, namely that this upregulation is mediated by nicotine exposure specifically through the α7 subtype of nicotine acetylcholine receptors (α7-nAChR). While exposure to increasing concentrations of nicotine caused epithelial cells to increase ACE-2 levels, subsequent gene silencing of α7-nAChR appeared to significantly dampen this response. A secondary transcriptome sequencing analysis of our cohort (consisting of 42 subjects who underwent bronchoscopy for epithelial cell brushings [1]) reveals evidence in support of this hypothesis.

α7-nAChR may upregulate ACE-2https://bit.ly/2xS0cfT

Amyloid-β, tau, and the cholinergic system in Alzheimer's disease: seeking direction in a tangle of clues

The link between histopathological hallmarks of Alzheimer's disease (AD), i.e. amyloid plaques, and neurofibrillary tangles, and AD-associated cognitive impairment, has long been established. However, the introduction of interactions between amyloid-beta (Aβ) as well as hyperphosphorylated tau, and the cholinergic system to the territory of descriptive neuropathology has drastically changed this field by adding the theory of synaptic neurotransmission to the toxic pas de deux in AD. Accumulating data show that a multitarget approach involving all amyloid, tau, and cholinergic hypotheses could better explain the evolution of events happening in AD. Various species of both Aβ and tau could be traced in cholinergic neurons of the basal forebrain system early in the course of the disease. These molecules induce degeneration in the neurons of this system. Reciprocally, aberrant cholinergic system modulation promotes changes in amyloid precursor protein (APP) metabolism and tau phosphorylation, resulting in neurotoxicity, neuroinflammation, and neuronal death. Altogether, these changes may better correlate with the clinical findings and cognitive impairment detected in AD patients. Failure of several of Aβ- and tau-related therapies further highlights the need for special attention to molecules that target all of these mentioned pathologic changes. Another noteworthy fact here is that none of the popular hypotheses of AD such as amyloidopathy or tauopathy seem to be responsible for the changes observed in AD alone. Thus, the main culprit should be sought higher in the stream somewhere in APP metabolism or Wnt signaling in the cholinergic system of the basal forebrain. Future studies should target these pathological events.

Strategies for the Treatment of Parkinson's Disease: Beyond Dopamine

Parkinson’s disease (PD) is the second-leading cause of dementia and is characterized by a progressive loss of dopaminergic neurons in the substantia nigra alongside the presence of intraneuronal α-synuclein-positive inclusions. Therapies to date have been directed to the restoration of the dopaminergic system, and the prevention of dopaminergic neuronal cell death in the midbrain. This review discusses the physiological mechanisms involved in PD as well as new and prospective therapies for the disease. The current data suggest that prevention or early treatment of PD may be the most effective therapeutic strategy. New advances in the understanding of the underlying mechanisms of PD predict the development of more personalized and integral therapies in the years to come. Thus, the development of more reliable biomarkers at asymptomatic stages of the disease, and the use of genetic profiling of patients will surely permit a more effective treatment of PD.

Alzheimer's Disease as a Membrane Disorder: Spatial Cross-Talk Among Beta-Amyloid Peptides, Nicotinic Acetylcholine Receptors and Lipid Rafts

Biological membranes show lateral and transverse asymmetric lipid distribution. Cholesterol (Chol) localizes in both hemilayers, but in the external one it is mostly condensed in lipid-ordered microdomains (raft domains), together with saturated phosphatidyl lipids and sphingolipids (including sphingomyelin and glycosphingolipids). Membrane asymmetries induce special membrane biophysical properties and behave as signals for several physiological and/or pathological processes. Alzheimer’s disease (AD) is associated with a perturbation in different membrane properties. Amyloid-β (Aβ) plaques and neurofibrillary tangles of tau protein together with neuroinflammation and neurodegeneration are the most characteristic cellular changes observed in this disease. The extracellular presence of Aβ peptides forming senile plaques, together with soluble oligomeric species of Aβ, are considered the major cause of the synaptic dysfunction of AD. The association between Aβ peptide and membrane lipids has been extensively studied. It has been postulated that Chol content and Chol distribution condition Aβ production and posterior accumulation in membranes and, hence, cell dysfunction. Several lines of evidence suggest that Aβ partitions in the cell membrane accumulate mostly in raft domains, the site where the cleavage of the precursor AβPP by β- and γ- secretase is also thought to occur. The main consequence of the pathogenesis of AD is the disruption of the cholinergic pathways in the cerebral cortex and in the basal forebrain. In parallel, the nicotinic acetylcholine receptor has been extensively linked to membrane properties. Since its transmembrane domain exhibits extensive contacts with the surrounding lipids, the acetylcholine receptor function is conditioned by its lipid microenvironment. The nicotinic acetylcholine receptor is present in high-density clusters in the cell membrane where it localizes mainly in lipid-ordered domains. Perturbations of sphingomyelin or cholesterol composition alter acetylcholine receptor location. Therefore, Aβ processing, Aβ partitioning, and acetylcholine receptor location and function can be manipulated by changes in membrane lipid biophysics. Understanding these mechanisms should provide insights into new therapeutic strategies for prevention and/or treatment of AD. Here, we discuss the implications of lipid-protein interactions at the cell membrane level in AD.

GTS-21 has cell-specific anti-inflammatory effects independent of α7 nicotinic acetylcholine receptors

α7 Nicotinic acetylcholine receptors (nAChRs) reportedly reduce inflammation by blocking effects of the important pro-inflammatory transcription factor, nuclear factor kappa-light chain-enhancer of B cells (NFκB). The α7 nAChR partial agonist GTS-21 reduces secretion of pro-inflammatory cytokines including interleukin-6 (IL6) and tumor-necrosis factor (TNF) in models of endotoxemia and sepsis, and its anti-inflammatory effects are widely ascribed to α7 nAChR activation. However, mechanistic details of α7 nAChR involvement in GTS-21 effects on inflammatory pathways remain unclear. Here, we investigate how GTS-21 acts in two cell systems including the non-immune rat pituitary cell line GH4C1 expressing an NFκB-driven reporter gene and cytokine secretion by ex vivo cultures of primary mouse macrophages activated by lipopolysaccharide (LPS). GTS-21 does not change TNF-stimulated NFκB signaling in GH4C1 cells expressing rat α7 nAChRs, suggesting that GTS-21 requires additional unidentified factors besides α7 nAChR expression to allow anti-inflammatory effects in these cells. In contrast, GTS-21 dose-dependently suppresses LPS-induced IL6 and TNF secretion in primary mouse macrophages endogenously expressing α7 nAChRs. GTS-21 also blocks TNF-induced phosphorylation of NFκB inhibitor alpha (IκBα), an important intermediary in NFκB signaling. However, α7 antagonists methyllycaconitine and α-bungarotoxin only partially reverse GTS-21 blockade of IL6 and TNF secretion. Further, GTS-21 significantly inhibited LPS-induced IL6 and TNF secretion in macrophages isolated from knockout mice lacking α7 nAChRs. These data indicate that even though a discrete component of the anti-inflammatory effects of GTS-21 requires expression of α7 nAChRs in macrophages, GTS-21 also has anti-inflammatory effects independent of these receptors depending on the cellular context.

The current agonists and positive allosteric modulators of α7 nAChR for CNS indications in clinical trials

The alpha-7 nicotinic acetylcholine receptor (α7 nAChR), consisting of homomeric α7 subunits, is a ligand-gated Ca²⁺-permeable ion channel implicated in cognition and neuropsychiatric disorders. Enhancement of α7 nAChR function is considered to be a potential therapeutic strategy aiming at ameliorating cognitive deficits of neuropsychiatric disorders such as Alzheimer's disease (AD) and schizophrenia. Currently, a number of α7 nAChR modulators have been reported and several of them have advanced into clinical trials. In this brief review, we outline recent progress made in understanding the role of the α7 nAChR in multiple neuropsychiatric disorders and the pharmacological effects of α7 nAChR modulators used in clinical trials.

Evaluating Commercially Available Antibodies for Rat α7 Nicotinic Acetylcholine Receptors

Alpha7 nicotinic acetylcholine receptors (α7 nAChRs) are important drug targets in neurological disorders and inflammation, making their detection and localization by validated antibodies highly desirable. However, tests in knockout animals raised questions about specificity of antibodies to mouse α7 nAChRs. To date, methods for validating antibodies for rat or human α7 nAChR have not been reported. We developed a gel-shift assay for western blots using GH4C1 cells expressing either native rat receptors or α7 nAChR-green fluorescent protein (GFP) chimeras to evaluate seven commercially available α7 nAChR antibodies. Blots with anti-GFP antibody detected GFP or α7 nAChR-GFP expressed in GH4C1 cells, and ¹²⁵I-α-bungarotoxin binding and RNA analysis demonstrated α7 nAChR expression. Validated samples were used to evaluate α7 nAChR antibodies by western blot and immunofluorescence studies. These methods confirmed that two of seven α7 nAChR antibodies identify gel-shifts for α7 nAChR/nAChR-GFP but only one antibody demonstrated low background and significant immunofluorescence differences between wild-type and α7 nAChR expressing GH4C1 cells. However, that polyclonal antibody displayed lot-to-lot variability. Our findings suggest that careful validation methods are required for all α7 nAChR receptor species and antibody lots and that the gel-shift assay may allow for relatively rapid antibody screening.

Nonmotor Symptoms in Experimental Models of Parkinson's Disease

Nonmotor symptoms of Parkinson's disease (PD) range from neuropsychiatric, cognitive to sleep and sensory disorders and can arise from the disease process as well as from drug treatment. The clinical heterogeneity of nonmotor symptoms of PD is underpinned by a wide range of neuropathological and molecular pathology, affecting almost the entire range of neurotransmitters present in brain and the periphery. Understanding the neurobiology and pathology of nonmotor symptoms is crucial to the effective treatment of PD and currently a key unmet need. This bench-to-bedside translational concept can only be successful if robust animal models of PD charting the genesis and natural history of nonmotor symptoms can be devised. Toxin-based and transgenic rodent and primate models of PD have given us important clues to the underlying basis of motor symptomatology and in addition, can provide a snapshot of some nonmotor aspects of PD, although the data are far from complete. In this chapter, we discuss some of the nonmotor aspects of the available experimental models of PD and how the development of robust animal models to understand and treat nonmotor symptoms needs to become a research priority.

Arrestins contribute to amyloid beta-induced cell death via modulation of autophagy and the α7nAch receptor in SH-SY5Y cells

Amyloid β-protein (Aβ) is believed to contribute to the development of Alzheimer’s disease (AD). Here we showed that Aβ_25-35 rapidly caused activation of autophagy, subsequently leading to reduction of autophagy associated with cellular apoptosis. Further investigation revealed that the accumulation of β-arrestin 1 (ARRB1) caused by Aβ_25-35 contributed to the induction of autophagic flux. The depletion of ARRB1 led to decreases in the expression of LC3B, Atg7, and Beclin-1, which are essential for the initiation of autophagy. ARRB1 depletion also reduced downstream ERK activity and promoted Aβ_25-35-induced cell death. As with ARRB1, transient upregulation of ARRB2 by Aβ_25-35 was observed after short treatment durations, whereas genetic reduction of ARRB2 caused a marked increase in the expression of the α7nAch receptor at the cell surface, which resulted in partial reversal of Aβ_25-35-induced cell death. Although expression of both ARRB1 and ARRB2 was reduced in serum from patients with AD, the levels of ARRB1 were much lower than those of ARRB2 in AD. Thus, our findings indicate that ARRB1/2 play different roles in Aβ_25-35 cytotoxicity, which may provide additional support for exploring the underlying molecular mechanism of AD.

Utilizing pHluorin-tagged Receptors to Monitor Subcellular Localization and Trafficking

Understanding membrane protein trafficking, assembly, and expression requires an approach that differentiates between those residing in intracellular organelles and those localized on the plasma membrane. Traditional fluorescence-based measurements lack the capability to distinguish membrane proteins residing in different organelles. Cutting edge methodologies transcend traditional methods by coupling pH-sensitive fluorophores with total internal reflection fluorescence microscopy (TIRFM). TIRF illumination excites the sample up to approximately 150 nm from the glass-sample interface, thus decreasing background, increasing the signal to noise ratio, and enhancing resolution. The excitation volume in TIRFM encompasses the plasma membrane and nearby organelles such as the peripheral ER. Superecliptic pHluorin (SEP) is a pH sensitive version of GFP. Genetically encoding SEP into the extracellular domain of a membrane protein of interest positions the fluorophore on the luminal side of the ER and in the extracellular region of the cell. SEP is fluorescent when the pH is greater than 6, but remains in an off state at lower pH values. Therefore, receptors tagged with SEP fluoresce when residing in the endoplasmic reticulum (ER) or upon insertion in the plasma membrane (PM) but not when confined to a trafficking vesicle or other organelles such as the Golgi. The extracellular pH can be adjusted to dictate the fluorescence of receptors on the plasma membrane. The difference in fluorescence between TIRF images at neutral and acidic extracellular pH for the same cell corresponds to a relative number of receptors on the plasma membrane. This allows a simultaneous measurement of intracellular and plasma membrane resident receptors. Single vesicle insertion events can also be measured when the extracellular pH is neutral, corresponding to a low pH trafficking vesicle fusing with the plasma membrane and transitioning into a fluorescent state. This versatile technique can be exploited to study localization, expression, and trafficking of membrane proteins.

Cholinergic System and Post-translational Modifications: An Insight on the Role in Alzheimer's Disease

BACKGROUND

Alzheimer's disease (AD) is the most common form of old age dementia. The formation of amyloid plaques (Aβ), neurofibrillary tangles and loss of basal forebrain cholinergic neurons are the hallmark events in the pathology of AD.

LITERATURE REVIEW

Cholinergic system is one of the most important neurotransmitter system involved in learning and memory which preferentially degenerates in the initial stages of AD. Activation of cholinergic receptors (muscarinic and nicotinic) activates multiple pathways which result in post translational modifications (PTMs) in multiple proteins which bring changes in nervous system. Cholinergic receptors-mediated PTMs "in-part" substantially affect the biosynthesis, proteolysis, degradation and expression of many proteins and in particular, amyloid precursor protein (APP). APP is subjected to several PTMs (proteolytic processing, glycosylation, sulfation, and phosphorylation) during its course of processing, resulting in Aβ deposition, leading to AD. Aβ also alters the PTMs of tau which is a microtubule associated protein. Therefore, post-translationally modified tau and Aβ collectively aggravate the neuronal loss that leads to cholinergic hypofunction.

CONCLUSION

Despite the accumulating evidences, the interaction between cholinergic neurotransmission and the physiological significance of PTM events remain speculative and still needs further exploration. This review focuses on the role of cholinergic system and discusses the significance of PTMs in pathological progression of AD and highlights some important future directions.

Molecular simulation study of the unbinding of α-conotoxin [ϒ4E]GID at the α7 and α4β2 neuronal nicotinic acetylcholine receptors

The α7 and α4β2 neuronal nicotinic receptors belonging to the family of ligand-gated ion channels are most prevalent in the brain, and are implicated in various neurodegenerative disorders. α-conotoxin GID (and its analogue [ϒ4E]GID) specifically inhibits these subtypes, with more affinity towards the human α7 (hα7) subtype, and is valuable in understanding the physiological roles of these receptors. In this study, we use umbrella-sampling molecular dynamics simulations to understand the mechanism of interaction between [ϒ4E]GID and the agonist binding pockets of the α4β2 and the hα7 receptors, and to estimate their relative binding affinities (ΔG_bind). The obtained ΔG_bind values indicate stronger interaction with the hα7 receptor, in agreement with previous experimental studies. Simulations also revealed different unbinding pathways between the two receptor subtypes, enabling identification of a number of interactions at locations far from the orthosteric binding site which may explain the difference in [ϒ4E]GID potency. The pathways identified will help in the design of novel conotoxins with increased potency at α4β2, for which there is currently no known highly potent conotoxin inhibitor. Computational mutational free energy analyses also revealed a number of possible single-site mutations to GID which might enhance its selective binding to α4β2 over α7.

Molecular changes in the postmortem parkinsonian brain

Parkinson disease (PD) is the second most common neurodegenerative disease after Alzheimer disease. Although PD has a relatively narrow clinical phenotype, it has become clear that its etiological basis is broad. Post-mortem brain analysis, despite its limitations, has provided invaluable insights into relevant pathogenic pathways including mitochondrial dysfunction, oxidative stress and protein homeostasis dysregulation. Identification of the genetic causes of PD followed the discovery of these abnormalities, and reinforced the importance of the biochemical defects identified post-mortem. Recent genetic studies have highlighted the mitochondrial and lysosomal areas of cell function as particularly significant in mediating the neurodegeneration of PD. Thus the careful analysis of post-mortem PD brain biochemistry remains a crucial component of research, and one that offers considerable opportunity to pursue etiological factors either by 'reverse biochemistry' i.e. from defective pathway to mutant gene, or by the complex interplay between pathways e.g. mitochondrial turnover by lysosomes. In this review we have documented the spectrum of biochemical defects identified in PD post-mortem brain and explored their relevance to metabolic pathways involved in neurodegeneration. We have highlighted the complex interactions between these pathways and the gene mutations causing or increasing risk for PD. These pathways are becoming a focus for the development of disease modifying therapies for PD. Parkinson's is accompanied by multiple changes in the brain that are responsible for the progression of the disease. We describe here the molecular alterations occurring in postmortem brains and classify them as: Neurotransmitters and neurotrophic factors; Lewy bodies and Parkinson's-linked genes; Transition metals, calcium and calcium-binding proteins; Inflammation; Mitochondrial abnormalities and oxidative stress; Abnormal protein removal and degradation; Apoptosis and transduction pathways. This article is part of a special issue on Parkinson disease.

Deficits in cholinergic neurotransmission and their clinical correlates in Parkinson's disease

In view of its ability to explain the most frequent motor symptoms of Parkinson’s Disease (PD), degeneration of dopaminergic neurons has been considered one of the disease’s main pathophysiological features. Several studies have shown that neurodegeneration also affects noradrenergic, serotoninergic, cholinergic and other monoaminergic neuronal populations. In this work, the characteristics of cholinergic deficits in PD and their clinical correlates are reviewed. Important neurophysiological processes at the root of several motor and cognitive functions remit to cholinergic neurotransmission at the synaptic, pathway, and circuital levels. The bulk of evidence highlights the link between cholinergic alterations and PD motor symptoms, gait dysfunction, levodopa-induced dyskinesias, cognitive deterioration, psychosis, sleep abnormalities, autonomic dysfunction, and altered olfactory function. The pathophysiology of these symptoms is related to alteration of the cholinergic tone in the striatum and/or to degeneration of cholinergic nuclei, most importantly the nucleus basalis magnocellularis and the pedunculopontine nucleus. Several results suggest the clinical usefulness of antimuscarinic drugs for treating PD motor symptoms and of inhibitors of the enzyme acetylcholinesterase for the treatment of dementia. Data also suggest that these inhibitors and pedunculopontine nucleus deep-brain stimulation might also be effective in preventing falls. Finally, several drugs acting on nicotinic receptors have proved efficacious for treating levodopa-induced dyskinesias and cognitive impairment and as neuroprotective agents in PD animal models. Results in human patients are still lacking.

Resistance to Inhibitors of Cholinesterase 3 (Ric-3) Expression Promotes Selective Protein Associations with the Human α7-Nicotinic Acetylcholine Receptor Interactome

The α7-nicotinic acetylcholine receptor (α7-nAChR) is a ligand-gated ion channel widely expressed in vertebrates and is associated with numerous physiological functions. As transmembrane ion channels, α7-nAChRs need to be expressed on the surface of the plasma membrane to function. The receptor has been reported to associate with proteins involved with receptor biogenesis, modulation of receptor properties, as well as intracellular signaling cascades and some of these associated proteins may affect surface expression of α7-nAChRs. The putative chaperone resistance to inhibitors of cholinesterase 3 (Ric-3) has been reported to interact with, and enhance the surface expression of, α7-nAChRs. In this study, we identified proteins that associate with α7-nAChRs when Ric-3 is expressed. Using α-bungarotoxin (α-bgtx), we isolated and compared α7-nAChR-associated proteins from two stably transfected, human tumor-derived cell lines: SH-EP1-hα7 expressing human α7-nAChRs and the same cell line further transfected to express Ric-3, SH-EP1-hα7-Ric-3. Mass spectrometric analysis of peptides identified thirty-nine proteins that are associated with α7-nAChRs only when Ric-3 was expressed. Significantly, and consistent with reports of Ric-3 function in the literature, several of the identified proteins are involved in biological processes that may affect nAChR surface expression such as post-translational processing of proteins, protein trafficking, and protein transport. Additionally, proteins affecting the cell cycle, the cytoskeleton, stress responses, as well as cyclic AMP- and inositol triphosphate-dependent signaling cascades were identified. These results illuminate how α-bgtx may be used to isolate and identify α7-nAChRs as well as how the expression of chaperones such as Ric-3 can influence proteins associating with α7-nAChRs. These associating proteins may alter activities of α7-nAChRs to expand their functionally-relevant repertoire as well as to affect biogenesis and membrane trafficking of α7-nAChRs.

Pharmacophore modeling, in silico screening, molecular docking and molecular dynamics approaches for potential alpha-delta bungarotoxin-4 inhibitors discovery

Background:

The alpha-delta bungartoxin-4 (α-δ-Bgt-4) is a potent neurotoxin produced by highly venomous snake species, Bungarus caeruleus, mainly targeting neuronal acetylcholine receptors (nAchRs) and producing adverse biological malfunctions leading to respiratory paralysis and mortality. Objective: In this study, we predicted the three-dimensional structure of α-δ-Bgt-4 using homology modeling and investigated the conformational changes and the key residues responsible for nAchRs inhibiting activity.

Materials and Methods:

From the selected plants, which are traditionally used for snake bites, the active compounds are taken and performed molecular interaction studies and also used for modern techniques like pharmacophore modeling and mapping and absorption, distribution, metabolism, elimination and toxicity analysis which may increase the possibility of success.

Results:

Moreover, 100's of drug-like compounds were retrieved and analyzed through computational virtual screening and allowed for pharmacokinetic profiling, molecular docking and dynamics simulation.

Conclusion:

Finally the top five drug-like compounds having competing level of inhibition toward α-δ-Bgt-4 toxin were suggested based on their interaction with α-δ-Bgt-4 toxin.

Cell-surface translational dynamics of nicotinic acetylcholine receptors

Synapse efficacy heavily relies on the number of neurotransmitter receptors available at a given time. In addition to the equilibrium between the biosynthetic production, exocytic delivery and recycling of receptors on the one hand, and the endocytic internalization on the other, lateral diffusion and clustering of receptors at the cell membrane play key roles in determining the amount of active receptors at the synapse. Mobile receptors traffic between reservoir compartments and the synapse by thermally driven Brownian motion, and become immobilized at the peri-synaptic region or the synapse by: (a) clustering mediated by homotropic inter-molecular receptor–receptor associations; (b) heterotropic associations with non-receptor scaffolding proteins or the subjacent cytoskeletal meshwork, leading to diffusional “trapping,” and (c) protein-lipid interactions, particularly with the neutral lipid cholesterol. This review assesses the contribution of some of these mechanisms to the supramolecular organization and dynamics of the paradigm neurotransmitter receptor of muscle and neuronal cells -the nicotinic acetylcholine receptor (nAChR). Currently available information stemming from various complementary biophysical techniques commonly used to interrogate the dynamics of cell-surface components is critically discussed. The translational mobility of nAChRs at the cell surface differs between muscle and neuronal receptors in terms of diffusion coefficients and residence intervals at the synapse, which cover an ample range of time regimes. A peculiar feature of brain α7 nAChR is its ability to spend much of its time confined peri-synaptically, vicinal to glutamatergic (excitatory) and GABAergic (inhibitory) synapses. An important function of the α7 nAChR may thus be visiting the territories of other neurotransmitter receptors, differentially regulating the dynamic equilibrium between excitation and inhibition, depending on its residence time in each domain.

Targeting brain α7 nicotinic acetylcholine receptors in Alzheimer's disease: rationale and current status

Alzheimer's disease (AD) is the most common form of dementia among older persons. Pathognomonic hallmarks of the disease include the development of amyloid senile plaques and deposits of neurofibrillary tangles. These changes occur in the brain long before the clinical manifestations of AD (cognitive impairment in particular) become apparent. Nicotinic acetylcholine receptors (AChRs), particularly the α7 subtype, are highly expressed in brain regions relevant to cognitive and memory functions and involved in the processing of sensory information. There is strong evidence that implicates the participation of AChRs in AD. This review briefly introduces current strategies addressing the pathophysiologic findings (amyloid-β-peptide plaques, neurofibrillary tangles) and then focuses on more recent efforts of pharmacologic intervention in AD, specifically targeted to the α7 AChR. Whereas cholinesterase inhibitors such as donepezil, galantamine, or rivastigmine, together with the non-competitive N-methyl-D-aspartate receptor antagonist memantine are at the forefront of present-day clinical intervention for AD, new insights into AChR molecular pharmacology are bringing other drugs, directed at AChRs, to center stage. Among these are the positive allosteric modulators that selectively target α7 AChRs and are aimed at unleashing the factors that hinder agonist-mediated, α7 AChR channel activation. This calls for more detailed knowledge of the distribution, functional properties, and involvement of AChRs in various signaling cascades-together with the corresponding abnormalities in all these properties-to be able to engineer strategies in drug design and evaluate the therapeutic possibilities of new compounds targeting this class of neurotransmitter receptors.

Mitogen-activated protein kinase signaling pathways are involved in regulating α7 nicotinic acetylcholine receptor-mediated amyloid-β uptake in SH-SY5Y cells

Intraneuronal accumulation of beta-amyloid protein (Aβ) is an early pathological change in Alzheimer's disease (AD). Recent studies demonstrate that α7 nicotinic acetylcholine receptor (α7nAChR) binds to soluble Aβ with a high affinity. In vitro and in vivo experiments also show that Aβ activates p38 MAPK and ERK1/2 signaling pathways via the α7nAChR. Interestingly, it has been reported that p38 MAPK and ERK1/2 signaling pathways affect the regulation of receptor-mediated endocytosis. These data suggest that MAPK signaling pathways maybe involved in the regulation of α7nAChR-mediated Aβ uptake. However, the evidence for this hypothesis is lacking. In the present study, we examined whether Aβ1-42 oligomers activate MAPK signaling pathways via α7nAChR, and assessed the role of MAPK signaling pathways in the regulation of Aβ1-42 uptake by α7nAChR. We confirm that undifferentiated SH-SY5Y cells are capable of taking up extracellular Aβ1-42. The internalization of Aβ1-42 accumulates in the endosomes/lysosomes and mitochondria. MAPK signaling pathways are activated by Aβ1-42 via α7nAChR. Aβ1-42 and α7nAChR are co-localized in SH-SY5Y cells and the expression of α7nAChR involves in Aβ1-42 uptake and accumulation in SH-SY5Y cells. Our data demonstrate that Aβ1-42 induces an α7nAChR-dependent pathway that relates to the activation of p38 MAPK and ERK1/2, resulting in internalization of Aβ1-42. Our findings suggest that α7nAChR and MAPK signaling pathways play an important role in the uptake and accumulation of Aβ1-42 in SH-SY5Y cells. Blockade of α7nAChR may have a beneficial effect by limiting intracellular accumulation of amyloid in AD brain and serves a potential therapeutic target for AD.

Methyllycaconitine- and scopolamine-induced cognitive dysfunction: differential reversal effect by cognition-enhancing drugs

There is a growing body of evidence pointing to the pivotal role of alpha-7 nicotinic acetylcholine receptor (α7 nAchR) dysfunction in cognitive disorders such as Alzheimer’s disease or schizophrenia. This study was undertaken to establish and characterize an in vivo model for cognitive disorder secondary to the blockade of α7 nAChR by its specific antagonist, methyllycaconitine (MLA). The results show that MLA elicited cognitive dysfunction as assessed by reduced spontaneous alternation of mice in the T-maze. The maximal effect of MLA produced 25–30% reduction in the spontaneous alternation of mice, a level comparable with that induced by the muscarinic antagonism of scopolamine. Donepezil and galantamine fully reversed both MLA and scopolamine-induced cognitive dysfunction. However, the ED₅₀ of donepezil and galantamine was significantly shifted to the left in the MLA- compared to scopolamine-treated mice (0.0005 and 0.002 mg/kg for donepezil; 0.0003 and 0.7 mg/kg for galantamine). Moreover, memantine elicited marked reversion of cognitive dysfunction (up to 70%) in MLA-treated mice while only a weak reversal effect at high dose of memantine (less than 20%) was observed in scopolamine-treated mice. The above findings indicate that MLA-induced cognitive dysfunction in the mouse is highly sensitive and more responsive to the current procognitive drugs than the traditional scopolamine-based assay. Thus, it can be of value for the preclinical screening and profiling of cognition-enhancing drugs.

Galantamine: use in Alzheimer’s disease and related disorders

Galantamine (Reminyl) has long been used as a traditional medicine and has an interesting pharmacology as it is both a reversible acetylcholinesterase inhibitor and an allosteric potentiator of nicotinic cholinergic receptors. The efficacy of galantamine has been studied in an extensive development program in Alzheimer's disease, and mixed and vascular dementia. Randomized, double-blind, placebo-controlled trials of up to 6 months duration and subsequent open-label follow-up studies have produced a broad spectrum of beneficial effects on cognitive and noncognitive disease symptoms. Apparent benefits for caregivers paralleled these, with a potentially reduced burden and cost. It appears that early and continued treatment maximizes the observed effects, which translate into economic terms, when applied to cost-effectiveness models. Overall, galantamine has a broad ranging efficacy and tolerability across an increasing range of conditions.

Brain excitability and connectivity of neuronal assemblies in Alzheimer's disease: from animal models to human findings

The human brain contains about 100 billion neurons forming an intricate network of innumerable connections, which continuously adapt and rewire themselves following inputs from external and internal environments as well as the physiological synaptic, dendritic and axonal sculpture during brain maturation and throughout the life span. Growing evidence supports the idea that Alzheimer's disease (AD) targets selected and functionally connected neuronal networks and, specifically, their synaptic terminals, affecting brain connectivity well before producing neuronal loss and compartmental atrophy. The understanding of the molecular mechanisms underlying the dismantling of neuronal circuits and the implementation of 'clinically oriented' methods to map-out the dynamic interactions amongst neuronal assemblies will enhance early/pre-symptomatic diagnosis and monitoring of disease progression. More important, this will open the avenues to innovative treatments, bridging the gap between molecular mechanisms and the variety of symptoms forming disease phenotype. In the present review a set of evidence supports the idea that altered brain connectivity, exhausted neural plasticity and aberrant neuronal activity are facets of the same coin linked to age-related neurodegenerative dementia of Alzheimer type. Investigating their respective roles in AD pathophysiology will help in translating findings from basic research to clinical applications.

Increased expression of the integral membrane proteins EGFR and FGFR3 in anti-apoptotic Chinese hamster ovary cell lines

Membrane proteins such as receptor tyrosine kinases (RTKs) have a vital role in many cellular functions, making them potential targets for therapeutic research. In this study, we investigated the coexpression of the anti-apoptosis gene Bcl-x(L) with model membrane proteins as a means of increasing membrane protein expression in mammalian cells. Chinese hamster ovary (CHO) cells expressing heterologous Bcl-x(L) and wild-type CHO cells were transfected with either epidermal growth factor receptor or fibroblast growth factor receptor 3. The CHO-Bcl-x(L) cell lines showed increased expression of both RTK proteins as compared with the wild-type CHO cell lines in transient expression analysis, as detected by Western blot and flow cytometry after 15 days of antibiotic selection in stable expression pools. Increased expression was also seen in clonal isolates from the CHO-Bcl-x(L) cell lines, whereas the clonal cell line expression was minimal in wild-type CHO cell lines. Our results demonstrate that application of the anti-apoptosis gene Bcl-x(L) can increase expression of RTK proteins in CHO cells. This approach may be applied to improve stable expression of other membrane proteins in the future using mammalian cell lines with Bcl-x(L) or perhaps other anti-apoptotic genes.

The nicotinic acetylcholine receptor: smoking and Alzheimer's disease revisited

Epidemiological studies regarding Alzheimer's disease (AD) in smokers currently suggest inconsistent results. The clinicopathological findings also vary as to how AD pathology is affected by smoking behavior. Even though clinicopathological, functional, and epidemiological studies in humans do not present a consistent picture, much of the in vitro data implies that nicotine has neuroprotective effects when used in neurodegenerative disorder models. Current studies of the effects of nicotine and nicotinic agonists on cognitive function in both the non-demented and those with AD are not convincing. More data is needed to determine whether repetitive activation of nAChR with intermittent or acute exposure to nicotine, acute activation of nAChR, or long-lasting inactivation of nAChR secondary to chronic nicotine exposure will have a therapeutic effect and/or explain the beneficial effects of those types of drugs. Other studies show multifaceted connections between nicotine, nicotinic agonists, smoking, and nAChRs implicated in AD etiology. Although many controversies still exist, ongoing studies are revealing how nicotinic receptor changes and functions may be significant to the neurochemical, pathological, and clinical changes that appear in AD.

α7 nicotinic ACh receptors as a ligand-gated source of Ca(2+) ions: the search for a Ca(2+) optimum

The spatiotemporal distribution of cytosolic Ca(2+) ions is a key determinant of neuronal behavior and survival. Distinct sources of Ca(2+) ions including ligand- and voltage-gated Ca(2+) channels contribute to intracellular Ca(2+) homeostasis. Many normal physiological and therapeutic neuronal functions are Ca(2+)-dependent, however an excess of cytosolic Ca(2+) or a lack of the appropriate balance between Ca(2+) entry and clearance may destroy cellular integrity and cause cellular death. Therefore, the existence of optimal spatiotemporal patterns of cytosolic Ca(2+) elevations and thus, optimal activation of ligand- and voltage-gated Ca(2+) ion channels are postulated to benefit neuronal function and survival. Alpha7 nicotinic -acetylcholine receptors (nAChRs) are highly permeable to Ca(2+) ions and play an important role in modulation of neurotransmitter release, gene expression and neuroprotection in a variety of neuronal and non-neuronal cells. In this review, the focus is placed on α7 nAChR-mediated currents and Ca(2+) influx and how this source of Ca(2+) entry compares to NMDA receptors in supporting cytosolic Ca(2+) homeostasis, neuronal function and survival.

Nicotine: specific role in angiogenesis, proliferation and apoptosis

Nowadays, tobacco smoking is the cause of ~5-6 million deaths per year, counting 31% and 6% of all cancer deaths (affecting 18 different organs) in middle-aged men and women, respectively. Nicotine is the addictive component of tobacco acting on neuronal nicotinic receptors (nAChR). Functional nAChR, are also present on endothelial, haematological and epithelial cells. Although nicotine itself is regularly not referred to as a carcinogen, there is an ongoing debate whether nicotine functions as a 'tumour promoter'. Nicotine, with its specific binding to nAChR, deregulates essential biological processes like regulation of cell proliferation, apoptosis, migration, invasion, angiogenesis, inflammation and cell-mediated immunity in a wide variety of cells including foetal (regulation of development), embryonic and adult stem cells, adult tissues as well as cancer cells. Nicotine seems involved in fundamental aspects of the biology of malignant diseases, as well as of neurodegeneration. Investigating the biological effects of nicotine may provide new tools for therapeutic interventions and for the understanding of neurodegenerative diseases and tumour biology.

Loss of alpha7 nicotinic receptors enhances beta-amyloid oligomer accumulation, exacerbating early-stage cognitive decline and septohippocampal pathology in a mouse model of Alzheimer's disease

Early Alzheimer's disease (AD) is marked by cholinergic hypofunction, neuronal marker loss, and decreased nicotinic acetylcholine receptor (nAChR) density from the cortex and hippocampus. alpha7 nAChRs expressed on cholinergic projection neurons and target regions have been implicated in neuroprotection against beta-amyloid (Abeta) toxicity and maintenance of the septohippocampal phenotype. We tested the role that alpha7 nAChRs perform in the etiology of early AD by genetically deleting the alpha7 nAChR subunit from the Tg2576 mouse model for AD and assessing animals for cognitive function and septohippocampal integrity. Thus, Tg2576 mice transgenic for mutant human amyloid precursor protein (APP) were crossed with alpha7 nAChR knock-out mice (A7KO) to render an animal with elevated Abeta in the absence of alpha7 nAChRs (A7KO-APP). We found that learning and memory deficits seen in 5-month-old APP mice are more severe in the A7KO-APP animals. Analyses of animals in early-stage preplaque cognitive decline revealed signs of neurodegeneration in A7KO-APP hippocampus as well as loss of cholinergic functionality in the basal forebrain and hippocampus. These changes occurred concomitant with the appearance of a dodecameric oligomer of Abeta that was absent from all other genotypic groups, generating the hypothesis that increased soluble oligomeric Abeta may underlie additional impairment of A7KO-APP cognitive function. Thus, alpha7 nAChRs in a mouse model for early-stage AD appear to serve a neuroprotective role through maintenance of the septohippocampal cholinergic phenotype and preservation of hippocampal integrity possibly through influences on Abeta accumulation and oligomerization.

Localisation of pre- and postsynaptic cholinergic markers in the human brain

The cholinergic neurotransmission in the central nervous system plays an important role in modulating cognitive processes such as learning, memory, arousal and sleep as well as in modulating locomotor activity. Dysfunction of the central cholinergic system is involved in numerous neuropsychiatric diseases. This review will provide a synopsis on the regional localisation of cholinergic and cholinoceptive structures within the adult human brain. On the cholinergic site data based on the distribution of choline acetyltransferase-immunoreactive structures are in the focus, complemented by data from acetylcholinesterase and vesicular acetylcholine transporter studies. On the cholinoceptive site, the distribution and localisation of receptors that transduce the acetylcholine message, i.e. the muscarinic and the nicotinic acetylcholine receptors is summarized. In addition to these data obtained on post mortem brain an overview of markers which allow for the in vivo monitoring of the cholinergic system in the brain is given. The detailed knowledge on the distribution and localisation of cholinergic markers in human brain will provide further information on the cholinergic circuits of neurotransmission - a prerequisite for the interpretation of in vivo imaging data and the development of selective diagnostic and therapeutic compounds.

Neuronal nicotinic acetylcholine receptor-cholesterol crosstalk in Alzheimer's disease

Alzheimer's disease (AD) is one of the most devastating diseases of the central nervous system (CNS). It is characterized by two neuropathological findings: amyloid plaques and neurofibrillary tangles. AD is also accompanied by an extensive functional deficit in the cholinergic system, involving the neuronal-type nicotinic acetylcholine receptor (AChR). Furthermore there is increasing evidence showing a misregulation of cholesterol metabolism in the development of the disease. Since cholesterol affects AChR protein at multiple levels, the cognitive impairment and other neurological correlates of AD might be partly associated with an abnormal crosstalk between the receptor protein and the sterol in this synaptopathy.

Alpha7 neuronal nicotinic receptors: the missing link to understanding Alzheimer's etiopathology?

Evidence supports the premise that alpha7 nicotinic acetylcholine receptors in the central nervous system, sometimes referred to as neuronal nicotinic receptors (NNRs), play a central role in the development of Alzheimer's disease (AD) pathophysiology. Moreover, these receptors may represent the key to unifying aspects of the cholinergic hypothesis of AD with many of the apparently disparate mechanisms such as beta-amyloid deposition, tau hyperphosphorylation, and ApoE4 abnormalities variously proposed to underlie the progression of the disease. We hypothesize that neuronal degeneration in incipient AD is the result of coincident events involving, at their core, deficits in alpha7 NNR function. The resulting hypocholinergic tone could potentially have serious consequences since alpha7 NNRs are known to modulate fundamental pathways involved in cell survival such as JAK2-STAT3. This hypothesis predicts that any factors that compromise alpha7 function have the potential to negatively impact neuronal viability. For example, such factors could include deficits in the primary neurotransmitter acetylcholine (ACh), underactivity of normal cognitive processes that stimulate alpha7 NNRs (i.e., use-dependency), or the reported binding of beta-amyloid and ApoE4 to alpha7 NNRs, all of which could effectively decouple the receptors from key pro-survival pathways. Since these pathways are known to negatively modulate GSK-3beta, which regulates tau phosphorylation, downstream effects such as tau hyperphosphorylation would be expected to arise. Conversely, the maintenance of normal alpha7 NNR activity by adequate levels of ACh or other NNR agonists would be expected to support normal cholinergic tone, prevent the binding of beta-amyloid and ApoE4 and preserve the integrity of the neurons. We therefore propose that decreased cholinergic tone is at the apex of AD pathophysiology, with factors such as beta-amyloid and ApoE4 playing a contributory role rather that a causal one and hyperphosphorylation of tau representing a detector of concomitant hypocholinergic tone and beta-amyloid deposition. Thus the convergence of beta-amyloid deposition and/or ApoE4 binding and co-localization with alpha7 NNRs, which are favored under conditions of low cholinergic tone, and the downstream cascade of tau hyperphosphorylation through disinhibition of GSK-3beta appear to explain and reconcile many of the discordant findings in this very active area of CNS research.

Nicotinic acetylcholine receptor signalling: roles in Alzheimer's disease and amyloid neuroprotection

Alzheimer's disease (AD), the major contributor to dementia in the elderly, involves accumulation in the brain of extracellular plaques containing the beta-amyloid protein (Abeta) and intracellular neurofibrillary tangles of hyperphosphorylated tau protein. AD is also characterized by a loss of neurons, particularly those expressing nicotinic acetylcholine receptors (nAChRs), thereby leading to a reduction in nAChR numbers. The Abeta(1-42) protein, which is toxic to neurons, is critical to the onset and progression of AD. The discovery of new drug therapies for AD is likely to be accelerated by an improved understanding of the mechanisms whereby Abeta causes neuronal death. We examine the evidence for a role in Abeta(1-42) toxicity of nAChRs; paradoxically, nAChRs can also protect neurons when activated by nicotinic ligands. Abeta peptides and nicotine differentially activate several intracellular signaling pathways, including the phosphatidylinositol 3-kinase/v-akt murine thymoma viral oncogene homolog pathway, the extracellular signal-regulated kinase/mitogen-activated protein kinase, and JAK-2/STAT-3 pathways. These pathways control cell death or survival and the secretion of Abeta peptides. We propose that understanding the differential activation of these pathways by nicotine and/or Abeta(1-42) may offer the prospect of new routes to therapy for AD.

Effects of cholinesterase inhibitors on rat nicotinic receptor levels in vivo and in vitro

Cholinesterase inhibitors (ChEIs) are the mainstay of treatment for AD but differ by secondary mechanisms of action. We determine the effects of sub-chronic dosing of ChEIs on alpha7 and non-alpha7 nAChRs and determine if differences can be observed between them. Sprague-Dawley rats were administered donepezil, galantamine; rivastigmine at two doses each, in saline SQ twice daily or with nicotine (0.4 mg/kg) as a positive control. After 14 days the animals were sacrificed, and the levels of nAChRs were measured using [3H]-EPI to measure non-alpha7 nAChRs and [3H]-MLA to measure alpha7 nAChRs. In the cortex, all compounds tested at the higher doses significantly increased the levels of both [3H]-EPI and [3H]-MLA. In the hippocampus all compounds significantly increased [3H]-EPI but had no effect on [3H]-MLA binding. No effects were observed in the striatum with treatment. There were no differences observed among the ChEIs. In cell cultures, none of the ChEIs increased non-alpha7 or alpha7 receptor binding. Treatment with ChEIs result in similar increases in receptor levels which suggest that the increases in nAChRs may be due simply to the increases in synaptic levels of acetylcholine.

Alpha7 nicotinic acetylcholine receptor expression by vascular smooth muscle cells facilitates the deposition of Abeta peptides and promotes cerebrovascular amyloid angiopathy

Deposition of beta-amyloid (Abeta) peptides in the walls of brain blood vessels, cerebral amyloid angiopathy (CAA), is common in patients with Alzheimer's disease (AD). Previous studies have demonstrated Abeta peptide deposition among vascular smooth muscle cells (VSMCs), but the source of the Abeta and basis for its selective deposition in VSMCs are unknown. In the present study, we examined the deposition patterns of Abeta peptides, Abeta40 and Abeta42, within the cerebrovasculature of AD and control patients using single- and double-label immunohistochemistry. Abeta40 and Abeta42 were abundant in VSMCs, especially in leptomeningeal arteries and their initial cortical branches; in later-stage AD brains this pattern extended into the microvasculature. Abeta peptide deposition was linked to loss of VSMC viability. Perivascular leak clouds of Abeta-positive material were associated primarily with arterioles. By contrast, control brains possessed far fewer Abeta42- and Abeta40-immunopositive blood vessels, with perivascular leak clouds of Abeta-immunopositive material rarely observed. We also demonstrate that VSMCs in brain blood vessels express the alpha7 nicotinic acetylcholine receptor (alpha7nAChR), which has high binding affinity for Abeta peptides, especially Abeta42. These results suggest that the blood and blood-brain barrier permeability provide a major source of the Abeta peptides that gradually deposit in brain VSMCs, and the presence and abundance of the alpha7nAChR on VSMCs may facilitate the selective accumulation of Abeta peptides in these cells.

The role of Wnt signaling in neuronal dysfunction in Alzheimer's Disease

Recent evidence supports a neuroprotective role for Wnt signaling in neurodegenerative disorders such as Alzheimer's Disease (AD). In fact, a relationship between amyloid-beta-peptide (Abeta)-induced neurotoxicity and a decrease in the cytoplasmic levels of beta-catenin has been observed. Apparently Abeta binds to the extracellular cysteine-rich domain of the Frizzled receptor (Fz) inhibiting Wnt/beta-catenin signaling. Cross-talk with other signaling cascades that regulate Wnt/beta-catenin signaling, including the activation of M1 muscarinic receptor and PKC, the use of Ibuprofen-ChE bi-functional compounds, PPAR alpha, gamma agonists, nicotine and some antioxidants, results in neuroprotection against Abeta. These studies indicate that a sustained loss of Wnt signaling function may be involved in the Abeta-dependent neurodegeneration observed in Alzheimer's brain. In conclusion the activation of the Wnt signaling pathway could be proposed as a therapeutic target for the treatment of AD.

Cholinergic and glutamatergic alterations beginning at the early stages of Alzheimer disease: participation of the phospholipase A2 enzyme

RATIONALE

Alzheimer disease (AD), a progressive neurodegenerative disorder, is the leading cause of dementia in the elderly. A combination of cholinergic and glutamatergic dysfunction appears to underlie the symptomatology of AD, and thus, treatment strategies should address impairments in both systems. Evidence suggests the involvement of phospholipase A(2) (PLA(2)) enzyme in memory impairment and neurodegeneration in AD via actions on both cholinergic and glutamatergic systems.

OBJECTIVES

To review cholinergic and glutamatergic alterations underlying cognitive impairment and neuropathology in AD and attempt to link PLA(2) with such alterations.

METHODS

Medline databases were searched (no date restrictions) for published articles with links among the terms Alzheimer disease (mild, moderate, severe), mild cognitive impairment, choline acetyltransferase, acetylcholinesterase, NGF, NGF receptor, muscarinic receptor, nicotinic receptor, NMDA, AMPA, metabotropic glutamate receptor, atrophy, glucose metabolism, phospholipid metabolism, sphingolipid, membrane fluidity, phospholipase A(2), arachidonic acid, attention, memory, long-term potentiation, beta-amyloid, tau, inflammation, and reactive species. Reference lists of the identified articles were checked to identify additional studies of interest.

RESULTS

Overall, results suggest the hypothesis that persistent inhibition of cPLA(2) and iPLA(2) isoforms at early stages of AD may play a central role in memory deficits and beta-amyloid production through down-regulation of cholinergic and glutamate receptors. As the disease progresses, beta-amyloid induced up-regulation of cPLA(2) and sPLA(2) isoforms may play critical roles in inflammation and oxidative stress, thus participating in the neurodegenerative process.

CONCLUSION

Activation and inhibition of specific PLA(2) isoforms at different stages of AD could be of therapeutic importance and delay cognitive dysfunction and neurodegeneration.

Wnt-7a induces presynaptic colocalization of alpha 7-nicotinic acetylcholine receptors and adenomatous polyposis coli in hippocampal neurons

Nicotinic acetylcholine receptors (nAChRs) contribute significantly to hippocampal function. Alpha7-nAChRs are present in presynaptic sites in hippocampal neurons and may influence transmitter release, but the factors that determine their presynaptic localization are unknown. We report here that Wnt-7a, a ligand active in the canonical Wnt signaling pathway, induces dissociation of the adenomatous polyposis coli (APC) protein from the beta-catenin cytoplasmic complex and the interaction of APC with alpha7-nAChRs in hippocampal neurons. Interestingly, Wnt-7a induces the relocalization of APC to membranes, clustering of APC in neurites, and coclustering of APC with different, presynaptic protein markers. Wnt-7a also increases the number and size of coclusters of alpha7-nAChRs and APC in presynaptic terminals. These short-term changes in alpha7-nAChRs occur in the few minutes after ligand exposure and involve translocation to the plasma membrane without affecting total receptor levels. Longer-term exposure to Wnt-7a increases nAChR alpha7 subunit levels in an APC-independent manner and increases clusters of alpha7-nAChRs in neurites via an APC-dependent process. Together, these results demonstrate that stimulation through the canonical Wnt pathway regulates the presynaptic localization of APC and alpha7-nAChRs with APC serving as an intermediary in the alpha7-nAChR relocalization process. Modulation by Wnt signaling may be essential for alpha7-nAChR expression and function in synapses.

Abeta peptides can enter the brain through a defective blood-brain barrier and bind selectively to neurons

We have investigated the possibility that soluble, blood-borne amyloid beta (Abeta) peptides can cross a defective blood-brain barrier (BBB) and interact with neurons in the brain. Immunohistochemical analyses revealed extravasated plasma components, including Abeta42 in 19 of 21 AD brains, but in only 3 of 13 age-matched control brains, suggesting that a defective BBB is common in AD. To more directly test whether blood-borne Abeta peptides can cross a defective BBB, we tracked the fate of fluorescein isothiocyanate (FITC)-labeled Abeta42 and Abeta40 introduced via tail vein injection into mice with a BBB rendered permeable by treatment with pertussis toxin. Both Abeta40 and Abeta42 readily crossed the permeabilized BBB and bound selectively to certain neuronal subtypes, but not glial cells. By 48 h post-injection, Abeta42-positive neurons were widespread in the brain. In the cerebral cortex, small fluorescent, Abeta42-positive granules were found in the perinuclear cytoplasm of pyramidal neurons, suggesting that these cells can internalize exogenous Abeta42. An intact BBB (saline-injected controls) blocked entry of blood-borne Abeta peptides into the brain. The neuronal subtype selectivity of Abeta42 and Abeta40 was most evident in mouse brains subjected to direct intracranial stereotaxic injection into the hippocampal region, thereby bypassing the BBB. Abeta40 was found to preferentially bind to a distinct subset of neurons positioned at the inner face of the dentate gyrus, whereas Abeta42 bound selectively to the population of large neurons in the hilus region of the dentate gyrus. Our results suggest that the blood may serve as a major, chronic source of soluble, exogenous Abeta peptides that can bind selectively to certain subtypes of neurons and accumulate within these cells.

A convergent model for cognitive dysfunctions in Parkinson's disease: the critical dopamine-acetylcholine synaptic balance

Parkinson's disease is classically characterised as a motor neurodegenerative disorder. Motor symptoms in the disorder are secondary to an altered dopamine-acetylcholine balance due to reduced striatal dopaminergic tone and subsequent cholinergic overactivity. In the past, anticholinergic drugs were given to improve motor aspects of the disease. There is now an increasing interest in the cognitive and non-motor symptoms of Parkinson's disease and in cholinesterase-inhibitor therapy for dementia associated with Parkinson's disease. In this Personal View, we reconsider the dopamine-acetylcholine balance theory and look at recent clinical findings and the possible cooperative role of dopamine and acetylcholine in the induction and maintenance of the long-lasting changes of striatal and cortical synaptic plasticity. We also discuss a convergent versus parallel model to explain cognitive dysfunctions in Parkinson's disease according to dopamine-acetylcholine dependent alterations in synaptic plasticity.

Role of the cholinergic system in regulating survival of newborn neurons in the adult mouse dentate gyrus and olfactory bulb

Neurogenesis in the subgranular zone of the hippocampal dentate gyrus and olfactory bulbs continues into adulthood and has been implicated in the cognitive function of the adult brain. The basal forebrain cholinergic system has been suggested to play a role in regulating neurogenesis as well as learning and memory in these regions. Herein, we report that highly polysialylated neural cell adhesion molecule (PSA-NCAM)-positive immature cells as well as neuronal nuclei (NeuN)-positive mature neurons in the dentate gyrus and olfactory bulb express multiple acetylcholine receptor subunits and make contact with cholinergic fibers. To examine the function of acetylcholine in neurogenesis, we used donepezil (Aricept), a potent and selective acetylcholinesterase inhibitor that improves cognitive impairment in Alzheimer's disease. Intraperitoneal administrations of donepezil significantly enhanced the survival of newborn neurons, but not proliferation of neural progenitor cells in the subgranular zone or the subventricular zone of normal mice. Moreover, donepezil treatment reversed the chronic stress-induced decrease in neurogenesis. Taken together, these results suggest that activation of the cholinergic system promotes survival of newborn neurons in the adult dentate gyrus and olfactory bulb under both normal and stressed conditions.